1. Field of the Invention
The present invention relates to specifically substituted polymerizable thioxanthones with high reactivity for LED exposure and increased thermal stability.
2. Description of the Related Art
Short run packaging printing is shifting from conventional printing techniques, such as offset printing, to digital printing, where inkjet is one of the preferred technologies. In inkjet printing, tiny drops of ink are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a print head for ejecting the drops image-wise on an ink-receiver. Within ink jet for digital packaging printing, there is a clear evolution towards higher image quality and higher printing speeds in combination with LED curing. In order to satisfy these demands, a new print head design is required. These print heads require a specific ink design as they only can operate with very low viscous inks. The inks for high resolution, high speed short run digital packaging printing have to combine low viscosity, low migrating properties after curing and high sensitivity for LED exposure.
Polymeric photoinitiators are known to improve the low migrating properties after curing, but also to increase the viscosity. Even polymeric photoinitiators with a compact design, like the hyperbranched photoinitiators disclosed by EP 1616921 A (AGFA), increase the viscosity of radiation curable compositions too much for the new print head designs.
Polymerizable photoinitiators are also known to improve the low migrating properties after curing. For example, EP 2161264 A (AGFA) discloses polymerizable Norrish Type II photoinitiators having a benzophenone group or a thioxanthone group in inkjet inks that exhibit low extractable amounts of the photoinitiators and their residues after curing.
Thioxanthones suitable for low migration radiation curable compositions have also been disclosed in EP 2444429 A (AGFA), EP 2199273 A (AGFA), GB 2454579 (LAMBSON), WO 03/033492 A (COATES BROTHERS), JP 2004224993 A (NIPPON KAYAKU) and EP 2246330 A (SIEGWERK).
High sensitivity for UV-LED exposure, preferably 395 nm LED exposure, requires bathochromic photoinitiators. Thioxanthones are known as being particularly preferred photoinitiators for LED exposure.
However, it has been observed that in very low viscous radiation curable compositions, the type of polymerizable photoinitiator also influences the thermal stability and shelf life upon storage, especially when they exhibit improved curing speed. Fluctuations in viscosity have a large impact on the jetting performance and reliability of the new print heads operating with very low viscous inkjet inks.
Therefore, there is still a need for highly reactive photoinitiators, especially for LED curing, with an improved stability performance in radiation curable formulations.